Method of chroma-keying for a digital video compression system

Abstract

A method of performing chroma-key coding including the steps of defining color regions and quantifying the number of pixels in the regions, classifying macroblocks in accordance with the number of pixels thus quantified, assigning chroma complexity weights for use in the computation of quantization step-size based on the classification of the macroblocks, computing the quantization step-size, performing quantization and performing variable length coding.

We claim: 
     
       1. A method of performing chroma-key coding, said method comprising the steps of: defining a color region around a color line and denoting the pixels lying within said color region as a key color and quantifying the number of said pixels as N k  ;   defining a background region around said color line and denoting the pixels lying outside said background region as foreground colors and quantifying the number of said pixels as N f  ;   defining all other colors that are neither said key color or said foreground colors as mixed colors and quantifying the number of said pixels representing said mixed colors as N m  ;   classifying macroblocks, said classification step comprising the substeps of: classifying a macroblock as a chroma-key color macroblock when (N T  -N k )÷N T  <ε, where ε is a predefined constant;   classifying a macroblock as a foreground object macroblock when (N T  -N f )÷N T  <ε;   classifying a macroblock as a transparency information macroblock when (N T  -N m )÷N T  <ε; and   classifying a macroblock as a boundary macroblock when said macroblock is not a chroma-key color macroblock, a foreground object macroblock or a transparency macroblock;     assigning a chroma complexity weight for use in the computation of quantization step-size and based on the classification of the macroblocks, said assigning step comprising the substeps of:   assigning a predetermined chroma complexity weight, WC macroblock  to the macroblock, said macroblock being one of a chroma-key color macroblock, foreground macroblock, transparency information macroblock and boundary block; and   computing quantization step-size based on said WC macroblock .   
     
     
       2. The method as claimed in claim 1, wherein said color is blue. 
     
     
       3. The method as claimed in claim 1, wherein said computing step includes the further steps of computing a normalization factor W N  and computing the normalized weight, W macroblock . 
     
     
       4. The method as claimed in claim 1, wherein said WC macroblock  is assigned a value of one (1) when the macroblock is one of a foreground macroblock and a chroma-key color macroblock. 
     
     
       5. The method as claimed in claim 4, wherein said WC macroblock  is assigned a value of 0.75 of the weight assigned to WC macroblock  of a foreground or chroma-key color macroblock when said macroblock is a transparency macroblock. 
     
     
       6. The method as claimed in claim 4, wherein said WC macroblock  is assigned a value of 0.50 of the weight assigned to WC macroblock  of a foreground or chroma-key color macroblock when said macroblock is a boundary macroblock. 
     
     
       7. A method of quantization based on the chromacity of a given macroblock, said method comprising the steps of: defining a color region around a color line and denoting the pixels lying within said color region as a key color and quantifying the number of said pixels as N k  ;   defining a foreground region around said color line and denoting the pixels lying outside said foreground region as foreground colors and quantifying the number of said pixels as N f  ;   defining all other colors that are neither said key color or said foreground colors as mixed colors and quantifying the number of said pixels representing said mixed colors as N m  ;   classifying macroblocks, said classification step comprising the substeps of: classifying a macroblock as a chroma-key color macroblock when (N T  -N k )÷N T  <ε, where ε is a predefined constant;   classifying a macroblock as a foreground object macroblock when (N T  -N f )÷N T  <ε;   classifying a macroblock as a transparency information macroblock when (N T  -N m )÷N T  <ε; and   classifying a macroblock as a boundary macroblock when said macroblock is not a chroma-key color macroblock, a foreground object macroblock or a transparency macroblock; and     assigning a chroma complexity weight for use in the computation of quantization step-size and based on the classification of the macroblocks, said assigning step comprising the substeps of:   assigning a predetermined chroma complexity weight, WC macroblock  to the macroblock, said macroblock being one of a chroma-key color macroblock, foreground macroblock, transparency information macroblock and boundary block; and   computing quantization step-size based on said WC macroblock .   
     
     
       8. The method as claimed in claim 7, wherein said color is blue. 
     
     
       9. The method as claimed in claim 7, wherein said color is red. 
     
     
       10. The method as claimed in claim 7, wherein said color is green. 
     
     
       11. The method as claimed in claim 7, wherein said computing step includes the further steps of computing a normalization factor W N  and computing the normalized weight, W macroblock . 
     
     
       12. The method as claimed in claim 7, wherein said WC macroblock  is assigned a value of one (1) when the macroblock is one of a foreground macroblock and a chroma-key color macroblock. 
     
     
       13. The method as claimed in claim 12, wherein said WC macroblock  is assigned a value of 0.75 of the weight assigned to WC macroblock  of a foreground or chroma-key color macroblock when said macroblock is a transparency macroblock. 
     
     
       14. The method as claimed in claim 12, wherein said WC macroblock  is assigned a value of 0.50 of the weight assigned to WC macroblock  of a foreground or chroma-key color macroblock when said macroblock is a boundary macroblock. 
     
     
       15. A method of performing chroma-key coding, said method comprising the steps of: defining a color region around a color line and denoting the pixels lying within said color region as a key color and quantifying the number of said pixels as N k  ; defining a background region around said color line and denoting the pixels lying outside said background region as foreground colors and quantifying the number of said pixels as N f  ; defining all other colors that are neither said key color or said foreground colors as mixed colors and quantifying the number of said pixels representing said mixed colors as N m  ;   classifying macroblocks, said classification step comprising the substeps of: classifying a macroblock as a chroma-key color macroblock when (N T  -N k )÷N T  <ε, where ε is a predefined constant; classifying a macroblock as a foreground object macroblock when (N T  -N f )÷N T  <ε; classifying a macroblock as a transparency information macroblock when (N T  -N m )÷N T  <ε; and classifying a macroblock as a boundary macroblock when said macroblock is not a chroma-key color macroblock, a foreground object macroblock or a transparency macroblock;   assigning a chroma complexity weight for use in the computation of quantization step-size and based on the classification of the macroblocks, said assigning step comprising the substeps of: assigning a predetermined chroma complexity weight, WC macroblock  to the macroblock, said macroblock being one of a chroma-key color macroblock, foreground macroblock, transparency information macroblock and boundary block; and   computing W N  and W macroblock  ;   computing the quantization step-size;   performing quantization; and   perform variable length coding.